The near field communication technology for contactless data exchange has been used in a variety of fields. Typically, the near field communication technology called Radio Frequency IDentification (RFID) is in widespread use. As one example, the RFID system is used in the Factory Automation (FA) field, such as quality control in a manufacturing process. More specifically, an RF tag which stores data such as identification information is fitted to an article that is a management target, or an object (pallet, container, and the like) for supporting or accommodating the article. Further, a communication device (hereinafter also referred to as a “reader-writer”) for exchanging data with the RF tag is disposed in the vicinity of a channel in which the article or the object fitted with the RF tag moves.
There are cases where an antenna of the reader-writer is installed in a position distant from the RF tag that is the communication target, and where the antenna is installed in a position near the RF tag that is the communication target, in accordance with facilities or a device for the installation.
In order to establish stable communication between the reader-writer and the RF tag, an installation margin of the reader-writer (a communicable distance to the RF tag (a communicable area range)) is preferably as large as possible. That is, it is preferable to install the antenna such that a difference between the maximum data receivable distance from the RF tab and the minimum data receivable distance from the RF tag is made as large as possible. The longer the distance between the antenna and the RF tag, the smaller a voltage of a reception signal (hereinafter referred from “reception voltage”) from the RF tag. The shorter the distance between the antenna and the RF tag, the larger the reception voltage from the RF tag. When a voltage dynamic range of a circuit for restoring the reception signal from the RF tag is narrow, the reception voltage is restricted to be either excessively low or excessively high. In order to deal with such a problem, a solution as described below has been proposed.
For example, Unexamined Japanese Patent Publication No. 2001-177435 discloses a contactless ID tag system. For coping with the problem of being unable to accurately read data due to a fixed amplification factor of a reception amplifier, this contactless ID tag system can discriminate and switch between a weak electric field and a strong electric field, to accurately receive ID data with excellent reception sensitivity.
Further, Unexamined Japanese Patent Publication No. 2013-062605 discloses a load modulation communication control device capable of keeping a dead zone small. More specifically, when confirming that bit determination has been failed despite the existence of an I-phase signal or a Q-phase signal, a reception error monitoring unit determines that a load modulation signal has been received but its reception intensity is excessively high. At this time, a resister setting unit rewrites a resister to lower a gain of a variable reception amplifier to lower amplitude of the load modulation signal in reception signal determination. With the reception amplifier gain in the low state, a communication re-execution unit re-executes the communication, to establish load modulation communication.